In a conventional rear swing arm installation structure 100, as shown in FIG. 7, pivot bearing holes 102c and 102d are coaxially drilled in a pair of vehicle body frames 102a and 102b. In addition, insertion holes 101c and 101d for a pivot bolt formed at the pivot sections of rear swing arms 101a and 101b are coaxially disposed between the pivot bearing holes 102c and 102d. Thereafter a pivot bolt 103 is inserted into the pivot bearing holes 102c and 102d and the insertion holes 101c and 101d and a nut 104 is screwed onto the pivot bolt 103. As a result, the rear swing arms 101a and 101b are in the state of being supported swingably around the pivot bolt 103 between the pair of the vehicle body frames 102a and 102b. See, for example, JP-A No. 122864/2004.
As illustrated in FIG. 7, the gap between the rear swing arm 101a and the vehicle body frame 102a is adjusted with an adjusting bolt 105a and a lock nut 106a. More specifically, the adjusting bolt 105a is screwed into the pivot bearing hole 102c of the vehicle body frame 102a, and the pivot bolt 103 is inserted into the adjusting bolt 105a. The adjusting bolt 105a moves along the axis line of the pivot bearing hole 102c by forward and backward movement that is driven by screwing the adjusting bolt 105a. Thus, the gap between the rear swing arm 101a and the vehicle body frame 102a is adjusted. The adjusting bolt 105a is fixed with the lock nut 106a in this state.
In addition, the gap between the rear swing arm 101b and the vehicle body frame 102b is adjusted with an adjusting bolt 105b and a lock nut 106b. More specifically, the adjusting bolt 105b is screwed into the pivot bearing hole 102d of the vehicle body frame 102b, and the pivot bolt 103 is inserted into the adjusting bolt 105b. The adjusting bolt 105b moves along the axis line of the pivot bearing hole 102d by forward and backward movement driven by screwing. Thus, the gap between the rear swing arm 101b and the vehicle body frame 102b is adjusted. The adjusting bolt 105b is fixed with the lock nut 106b in this state.
However, in the rear swing arm installation structure 100, the gaps between the rear swing arms 101a and 101b and the vehicle body frames 102a and 102b are adjusted with the adjusting bolts 105a and 105b and the lock nuts 106a and 106b. In the case of a motorcycle with a shaft drive mechanism, the weight of the rear swing arms 101a and 101b is substantial. Thus, problems arise wherein the adjusting bolts 105a and 105b require a large tightening torque. Thereby, an upsizing of the lock nuts 106a and 106b is required which results in an increase in the number of manufacturing processes and an increase in the weight.